The present invention relates generally to liquid crystal devices used in the recording of image information. More particularly, the invention relates to a liquid crystal cell upon which information can be written by a laser beam for subsequent viewing or projection.
Liquid crystal materials have received considerable attention over the past several decades as a result of the ability of such materials to be relatively easily transformed from a transparent state to a light-scattering state. Consequently, such materials have found use in the recording and/or display of image information.
Liquid crystal materials can generally be divided into several classes. One such class, known as smectic liquid crystals, possess a storage capability in that image information need be written onto a liquid crystal cell only once. The scattering regions which are created in recording the information are essentially static, and the written information is thereafter essentially permanent until erased.
Laser-addressed liquid crystal cells have been developed utilizing these smectic materials as high resolution projection and/or display devices. The writing mechanism on these devices is primarily thermal. A focused infrared laser beam is used to heat the smectic liquid crystal material into the isotropic state. Afterwards, the liquid crystal is cooled back to smectic state and forms a light-scattering region. The written scattering region is stable within the smectic temperature range of the specific liquid crystal material, and the written information will be preserved. The cell can be erased by an electric field or by a combined effect of heating and applied electric field.
The image information written onto the cell can be viewed by an observer of the cell. Also, and perhaps more importantly, visible light may be directed onto or through the cell following writing of the information, and any image can be projected onto a display screen, a photosensitive material, or the like. A variety of such devices are discussed in Dewey, "Laser-Addressed Liquid Crystal Displays," Optical Engineering 23(3), 230-240 (May/June 1984).
Two types of laser-addressed liquid crystal cells have been developed for use in conjunction with an infrared diode laser. Both are discussed in the referenced publication of Dewey. One such device, known as a reflective device, utilizes a thin film infrared absorber fabricated on one of the substrates of the liquid crystal cell. As the infrared beam is scanned across the absorber layer, the radiation is converted to heat to produce scattering regions within the cell. However, the thin film infrared absorber is opaque, not only to infrared radiation but also to the visible radiation used in projecting the finished image. Thus, the image written on the liquid crystal cell must be projected by reflecting the visible radiation off the cell.
A second type of device is commonly known as a transmissive device. Typically, an infrared absorption dye having its peak absorption at the laser wavelength is doped into the liquid crystal material. As the writing beam is scanned across the cell, the dye absorbs the laser radiation and converts it to heat. The dye has little or no effect on light of a visible wavelength. Thus the image written on such a device can be projected simply by passing projection light through the device in a manner similar to that used in an ordinary slide projector.
An alternative arrangement for a transmissive device is shown in copending, commonly-assigned application Ser. No. 53,216, filed May 22, 1987. A three-layer structure absorptive of the infrared writing beam but transparent to visible light is incorporated into the cell. The writing beam thus acts to create the scattering regions in the liquid crystal necessary to record image information. However, the visible projection beam passes with relatively low attenuation through the absorptive structure for projection of the image.
In any case, the image information is typically written onto the cell as a series of very small marks or dots which together comprise the complete image. The beam is scanned over the cell and modulated to produce the dots. One advantage to such an approach is that since the image information to be written is typically processed as a series of pixels, writing the information as a series of dots simplifies the manipulation of data. However, the writing beam must be precisely controlled in order to position the dots properly across the cell. Variations in scan rate or the accuracy of beam modulation can produce distortions, patterning, loss of resolution or other loss of information in the image output. Of course, the scanning mechanisms needed to avoid this situation are both complex and costly.
What is needed, therefore, is a means by which the liquid crystal cell can be addressed in a precise manner by a laser beam. The high degree of precision with which the cell is addressed by the laser must be both consistent and repeatable. Further, such an addressing means should be capable of such performance at reasonable cost and complexity.